The present invention relates to methods of reel-laying a mechanically lined pipe (MLP), particularly but not exclusively to provide a submarine pipeline; to so-formed and laid MLPs; and to methods of manufacture of such MLPs. It relates also to the MLP itself which is specifically designed to be used in such reel laying methods, including those described in Standard API 5L/ISO 3183:2007 for seamless and welded steel pipes.
Corrosion resistance pipelines for the submarine or otherwise underwater transportation or conveying of corrosive fluids such as gas or crude oil can be provided by pipes having an internal metallic liner. A double-walled or bi-metallic pipe is generally composed of two metallic layers. The outer layer is to provide resistance against buckling on the reel or sea bottom and provides general strength to the design so as to resist to hydrostatic pressure, whilst the internal layer protects the outer layer from damage due to the chemical composition of the fluid being conveyed. The inner layer is sometimes also termed a “liner”. As the main purpose is to protect the outer layer from corrosion, a corrosion resistant alloy (CRA) is commonly chosen as the liner.
One form of bi-metallic pipe is a single “clad” pipe having an internal CRA layer metallurgically bonded to the outer layer, which could be formed from a carbon steel base metal.
A second form of bi-metallic pipe can be termed a mechanically lined pipe (MLP), where a liner is fixed to the outer layer (such as carbon steel) without metallurgical bonding. An economic method of forming a lined pipe uses hydraulic and/or mechanical expansion, where the liner is inserted into the outer layer, and then both parts are expanded. During the expansion, the inner pipe undergoes a plastic deformation while the outer layer undergoes either an elastic or a plastic deformation, depending on the manufacturing process. One example of this comprises inserting an alloy 316L liner inside a carbon steel host pipe, and expanding the liner radially so that it comes into contact with the host pipe, and then the host pipe outer diameter will also expand together with the liner to a pre-determined strain level such that, following relaxation of the internal pressure, an interference contact stress between the liner and the host pipe remains.
There are two common methods of laying underwater or submarine pipelines. The so-called ‘stove piping method’ involves assembling pipe stalks on a pipe-laying vessel, and then welding each one as the laying progresses. In the so-called ‘reeled laying method’, the pipeline is assembled onshore and spooled onto a large reel, sometimes also termed a storage reel or drum. Once offshore, the pipeline is spooled off from the reel, straightened and/or aligned and finally laid on the seabed. So, no welding is required during the offshore operation, saving time for the vessel operation
The reeled laying method is faster and more economical than the stove piping method, such that it is preferred where possible. However, the reeling process obviously generates significant multiple bending strains in the pipeline, which would cause a conventional 2.5-3.0 mm liner in a conventional lined pipe to wrinkle, and it is currently considered that wrinkles are detrimental to an MLP. Thus, all current methods developed to load an MLP onto a reel have been based on the idea that the formation of any wrinkles should be avoided at all costs during the spooling processes.
For this reason, there are currently no commercial applications of the reeled lay method for bi-metallic pipes, and any methods proposed to date for spooling an MLP onto a reel involve maintaining a pressure inside the pipeline during each step of the reeling/unreeling process to avoid the formation of wrinkles. However, a significant volume of pressurized gas will significantly increases the risk of serious injury in case of pipeline failure during reeling/unreeling operations.
For example, WO 2008/072970 A1 discloses a method for laying a pipeline having an inner corrosion proof metallic liner that is held inside an outer pipe material by interference stresses. In its method, a section of the pipeline is reeled onto a pipe laying drum, whilst an overpressure is maintained within the section by means of a pressurised fluid. When the pipeline is motionless, the overpressure is relieved, and a further pipeline section is joined to the first section. A new overpressure is then applied within the sections, and the further section is reeled onto the pipe laying drum.
Whilst this method may assist to avoid wrinkling when the pipeline sections have “mechanical movement” (defined in WO 2008/072970 A1 as meaning reeling the pipeline onto or unwinding the pipeline from the pipe laying drum), this method requires the overpressuring and pressure-relieving steps every increment when two pipe sections are joined. The pipe laying drum is described in WO 2008/072970 A1 as typically having installed “many” pre-fabricated sections, creating significant repetition of the overpressuring and pressure-relieving steps required.